BIOL 212: The Diversity of Life —Where should we start?


One of the most important outcomes of modern biology is the demonstration of the unity of life

3 domain hypothesis: Linking Prokaryotes and Eukaryotes


















In 1857, Charles Darwin sent a letter to Thomas Huxley in which he wrote: “The time will come I believe, though I shall not live to see it, when we shall have very fairly true genealogical trees of each great kingdom of nature.”

3 domains are not universally accepted…



  • 1990, Carl Woese proposed that all cellular life could be placed into 1 of 3 ‘domains’
    • Bacteria, Archaea and Eukarya
    • comparisons of small RNA sequences
    • Eukarya domain more closely related to Archaea


  • New genomic data argue that eukaryotes and archaea should not be separate


  • The debate is unresolved and ongoing…

Is it 2 or 3 domains? That is the question…

Looking for LUCA: Ancient Genes

  • The nature of the earliest ancestor of all living things has long been uncertain
    • domains of life seem to have no common point of origin

  • Ancestor was a single-cell, bacterium-like organism
    • Last Universal Common Ancestor (LUCA)
    • ~4 billion years ago


  • Scientists search for ancestral genes related by common descent
    • 355 genes point to an organism that lived in the conditions found in deep sea vents


  • Vent organism is missing genes necessary for life
    • must have been relying on chemical components from its environment
    • “half-alive”

Prokaryotes first to inhabit the Earth

Prokaryotes matter: First fossils



  • First TRACES of living organisms
    • Graphite carbon from Canadian rocks
    • Formed by ‘prokaryotes’
    • ~ 3.9 billion years ago!


  • Cyanobacteria form stromatolites
    • undisputed fossils = 3.5 bya
    • western Australia
    • layers of calcium carbonate precipitated over bacterial colonies

Common ancestor of all species living today: Prokaryote

Prokaryotes: The big picture


  • ‘Prokaryotes’ are not a monopheyltic group
    • Kingdoms Bacteria & Archaea; each with unique traits
    • most species diverse groups on Earth


  • Diversity of ways of making a living
    • sources of energy to make ATP
    • heterotrophs
    • autotrophs (photosynthesis)


  • Co-evolution with the planet and other organisms
    • oxygen poor to oxygen rich world
    • extremeophiles (temperature, pH, salt,etc.)
    • prokaryotes are most abundant organisms on Earth!

Cells of Prokaryotes are simpler than Eukaryotes: Brain Storm

Most Prokaryotes have a cell wall


  • Maintains cell shape, protects cells and prevents it from bursting in hypotonic solution


  • In hypertonic environments prokaryotes loss water and shrink away from wall
    • an halt reproduction
    • why salt is used to preserve food!


  • Major component of the bacterial cell wall is peptidoglycan
    • rigid structure
    • specific only to prokaryotes

Cell surface structures: Gram + & - bacteria

Hospitals: Gram staining as a fast diagnostic tool


  • 2 main categories of bacterial infections
    • Gram-positive and Gram-negative


  • Gram stain is colored purple
    • If stays purple = Gram-positive
    • If turns pink = Gram-negative


  • Gram+ : MRSA, strep and toxic shock.


  • Gram- : salmonella, pneumonia, UTI’s, and gonorrhea.


Gram - bacteria are more antibiotic resistance

Evolution of traditional groups:


  • Gram positive bacteria are monophyletic


  • Cyanobacteria are monophyletic


  • Gram negative bacteria no longer considered monophyletic

Prokaryotes are structurally diverse


  • Unicellular (some colonies)


  • Small
    • microscopic to poppy seed


  • Many Shapes
    • spherical, rod, spiral


  • 1/2 are motile (taxis)
    • use flagellum

The origins of movement: Flagella


  • 3 main structures: motor, hook and filament (tail)
    • relatively fast
    • very different from eukaryotes
    • complexity often used in creationism arguments


  • 19 shared proteins are modified versions of existing proteins
    • protein functions adapted to take on new functions
    • through selection!


  • Allows for chemotaxis, direct movement in response to chemicals
    • towards or away

Next time: Prokaryote functional diversity and importance